Some luminescent substances (phosphors), which comprise an inorganic compound (host) generally activated with elemental ions (activators), emit light (which is usually visible light but may be also infrared or UV radiation and is generally called soft or low energy radiation) upon exposure to hard or high energy radiation, such as X-rays, cathode rays, short UV rays.
The soft radiation emission may be either a direct emission, i.e. the phosphor emits light while it is still under irradiation (direct phosphor), or a photostimulated emission, i.e. the irradiated phosphor emits light if stimulated by a suitable stimulation light (stimulable phosphor).
Direct emission phosphors find application in intensifying screens for radiography in which the relevant luminescence of the phosphor is selected to be in the band of higher sensitivity of either a silver halide photographic film (direct radiography) or a photodetector (digital radiography) which are coupled to the phosphor screen for detecting the emitted light. The X-ray intensifying screen absorbs the X-rays and converts the X-rays, through luminescence, into light energy to which the photographic film or the photodetector are sensitive. Screens absorb a much larger fraction of X-rays than photographic films and photodetectors do and thus are used in order to reduce the X-ray dosage.
Among direct emission phosphors known in the art, the most efficient are generally considered the gadolinium oxy-sulfide phosphors activated with terbium (Gd.sub.2 O.sub.2 S:Tb), the barium fluoro-choride phosphors activated with europium (BaFCI:Eu), the calcium tungstate (CaWO.sub.4), the yttrium tantalate phosphors activated with niobium (YTaO.sub.4 :Nb), the gadolinium tantalate phosphors activated with terbium(GaTaO.sub.4 :Tb) and the rare earth silicate phosphors activated with terbium and/or cerium.
The X-ray intensifying screens are made of relatively thick layers of X-ray absorbing phosphors which convert X-rays into light energy. Generally, X-ray intensifying screens comprise a support having coated thereon a phosphor layer which comprises phosphor particles dispersed into a suitable binder in a phosphor-to-binder volume ratio between about 0.8/1 to about 4/1 and phosphor coverages between about 100 g/m.sup.2 and about 2000 g/m.sup.2, as described for example in Research Disclosure, vol. 184, item 18431, August 1979.
The efficiency of an X-ray intensifying screen (i.e., the absorption rate of radiation and the conversion rate to light) is essentially determined not only by the emission luminescence of the phosphor per se but also by the content of the phosphor in the phosphor layer, and consequently requires the layer to be very thick. The increased thickness of the phosphor layer results also in an increase of the image quality (reduced graininess). On the other hand, in the X-ray intensifying screens, the thicker the phosphor layer is, the less sharp the image tend to be due to the spread of emitted light. Thus, so far as the thickness of the layer containing the phosphor is concerned, the efficiency or graininess of the resulting image versus the sharpness of the resulting image present opposite requirements. Further, it is known that a high relative density (proportion by volume of the phosphor occupying the phosphor layer) can improve the efficiency/resolution characteristics of an X-ray intensifying screen. However, such improvement is typically limited in a powder phosphor layer by the intergrain volume (usually occupied by the organic binder), that is generally about 50 per cent of the overall layer volume. Further, when the phosphor layer having the binder contains a great number of air bubbles, the emitted light tends to scatter.
Dense binderless phosphor layers, with an intergrain volume near to zero, and thus having a relative density near to that of the bulk phosphor, appear to constitute the most convenient arrangement in X-ray intensifying screens.
Methods of increasing the relative density of a phosphor layer using binderless phosphor layers are known in the art. Said methods comprise 1) methods of compressing a phosphor layer using compression means such as a calendar roll or a hot press (U.S. Pat. No. 3,859,527), 2) methods in which the phosphor layer is formed by firing (JP 61-73100 and JP 59-196365), 3) sintering methods (EP253,348: stimulable phorphors), and 4) methods of forming a binderless phosphor layer in which said layer is deposited onto a support by sputtering, vacuum evaporation or chemical vapor deposition (EP 175,578 and EP 230,314: stimulable phosphors, and EP 322,715: activated ZnS or (Zn,Cd)S).
The above methods of manufacturing binderless phosphor layers, such as sintering, vacuum evaporation or sputtering, have not been found suitable for manufacturing binderless layers of direct emission phosphors, which have melting points higher than 1800.degree. C. and low vapor tensions, such as rare earth oxysulfides, rare earth silicates and rare earth tantalates.
Further examples of methods of manufacturing binderless phosphor layers comprise 1) vacuum evaporation and subsequent heat treatment of ZnS(Mn), Zn.sub.2 SiO.sub.4 (Mn), Zn.sub.2 (PO.sub.4).sub.3 (Mn), CaF.sub.2 (Mn) and CaWO.sub.4 (W) phosphors (C. Feldman and M. O'Hara, J. Opt. Soc. Am., V. 47, p. 300-305, April 1957), 2) chemical vacuum deposition of activated ZnS (F. Studer and D. Cusano, J. Opt. Soc. Am., V. 45, p. 493-497, July 1955), 3) gelling techniques in systems containing CaO, MgO, Al.sub.2 O.sub.3, SiO.sub.2, Fe--O.sub.2 and Na.sub.2 O (G. Biggar and M. O'Hara, V. 37, p. 198-205, June 1969), 4) sol-gel techniques in terbium doped yttrium silicates (E. Rabinovich et al., Am. Ceram. Soc. Bull., V. 66, p. 1505-1509, 1987), 5) growth by liquid phase epitaxy of cerium activated yttrium aluminum oxide phophors (G. Berkstresser et al., J. Electrochem. Soc., V. 134, p. 2624-2628, October 1987), and 6) electron beam evaporation method for the synthesis of BaFBr:Eu phosphor film (H. Kobayashi et al., J. Lumines., V. 40/41, p. 819-820, 1988).
A solution spray pyrolysis technique has been described for preparation of very thin and transparent luminescent films of activated zinc and cadmium silicates (R. D. Kirk and J. H. Schulman, J. Electrochem. Soc., V. 108, p. 455-457, May 1961),
Such methods, however, have been found to be not desirable nor convenient for manufacture of thick binderless layers of luminescent substances having the sensitivity and image sharpness required in X-ray intensifying screens, in particular those X-ray intensifying screens using rare earth activated rare earth silicate direct phosphors.